T-buffer Investigated - Page 2

Published on 26th Nov 1999, written by Kristof Beets and Dave Barron for Consumer Graphics - Last updated: 25th May 2007

If you need a real world example of this try watching a video film using
your VCR. Notice that the playback looks smooth and even objects look
sharp. Now press the pause-button, you should notice that the still image
is blurry and even un-sharp. This is not because you have a bad VCR (well
it could be), its just caused by the fact that each image is a summation
of a large amount of sharp pictures, this summation looks blurry. But
if you play this animation back you get an almost perfect reproduction
of what happens in the real world. You play summations over time during
1/30th of a second back at 30 frames per second, thus perfect (in theory,
reality suffers from quite a few bugs, but that's not the subject of this
article) reproduction of the real world motion. The series of pictures
is a poor approximation since it shows still images, images that are a
snapshot in time. If you play those back at 30fps then you miss a lot
of info. Basically between each frame a lot of temporal information is
missing and as a result you get the stop-and-go effect.

Now, all those effects have names. The summation over time I have been
talking about its generally referred to as motion blur. Motion blur is
generally not associated with video cameras, it's most known from photography.
You can take photos with a very long shutter time (up to many seconds).
By using a long shutter time you get an exaggerated situation of what
I described before, and as a result everything that moves looks like a
smeared out mess. I am sure you have seen pictures like that taken from
a highway with cars at night. All the headlights are turned into long
white lines. These lines are the summation over time of the positions
of the headlights.

So what is the link with 3dfx's T-Buffer?

T-Buffer technology tries to solve the problem I described above. Instead
of having a single isolated perfect snapshot in time, we are trying to
recreate the summation over time created automatically by a Video Camera.
If we can simulate this summation over time we should see an increased
quality of the animations generated by the 3D card. Now, how can we generate
such a summation over time in single frame?

The easiest way to do this is by simply combining several rendered images
into one single frame. Now, at this point, we have to decide what we want:
more true frames per second (being true perfect snapshots shown) or a
combination of several images (sub frames) in one single frame. To help
us decide we have to known that the human eye and brain combination is
capable of identifying at least 30 individual frames per second. This
means that if you are shown 30 frames/sec you can identify them as being
separate images (there are special tests done to come to this conclusion.
For example, you are shown dot patterns at a rate of 30fps and at the
end you have to say whether a certain pattern was present in the series
or not. if you move above 30fps you see several of those patterns as one
making it impossible to identify whether a pattern was present or not,
while at frame rates of 30fps and below you can identify the individual
image contents and thus give the correct answer). What happens is that
if you increase the frame rate the brain starts to see multiple frames
as one single frame. So, what happens is that our eyes and brain add something
similar to motion blur (real world sample: move your hand quickly left
to right in front of your face notice how the hand looks "smeared" and
"blurry"). The problem is that different people get this effect at different
frame rates. While, for some people, this brain-induced "motion blur"-effect
kicks in at 30fps it might kick in at 70fps for others. So, instead of
relying on the brain to combine images and create motion blur, we could
use the computer to combine images and thus create "fake" motion blur.
What T-Buffer technology actually does is it takes several images rendered
and combines them into one single frame. By doing this, the brain gets
confused, and it suddenly sees a motion blurred image. The brain links
this effect with speed and the feeling of realism is increased dramatically.
So, it's very possible that an animation at 30fps using motion blur looks
much more realistic than a perfect, still frame animation at 60fps simply
due to the effect that the brain isn't fooled so easily to add motion
blur at 60fps.